This invention relates to an improved method and apparatus for measuring the contact area of one or more electrosurgical electrodes or a cryosurgical probe with respect to a patient's tissue.
In monopolar electrosurgical systems, it is well known to effect cutting, fulguration or desiccation by employing an active electrode having a relatively small cross-sectional area whereby the current density at the treated area is sufficiently high to effect the desired treatment. Assuming the current density is sufficiently high, desiccation occurs if the active electrode is in ohmic contact with the tissue. Assuming the voltage is sufficiently high, cutting or fulguration occurs depending on the type of waveform if the active electrode is in arcing contact with the tissue. A large patient electrode is also normally employed in monopolar electrosurgery to return the current to the electrosurgical generator, the surface area of this electrode being large enough to reduce the current density threat to a point where the possibility of a patient burn is minimal.
There is a possibility the patient may not be in contact with the patient electrode or that a discontinuity may arise in the return circuit from the patient electrode to the generator. If a small grounded object is in contact with the patient at this time, the current from the active electrode will return to the generator through an alternate return path including the small grounded contact point whereby the possibility of a patient burn at the contact points exists. Circuits are known for detecting alternate return ground currents of the foregoing type and effecting appropriate remedial procedures whenever the alternate return current exceeds a predetermined minimum, see for example, U.S. Pat. No. 3,683,923.
Another potential danger exists in that the patient may only be in partial contact with the patient electrode due, for example, to a misapplication of the electrode. The remaining area of contact may be of insufficient size to sufficiently reduce the current density at the return electrode to prevent burning of the patient threat. Safety circuitry is known whereby split (or double) patient electrodes are employed and a DC current (see German Pat. No. 1,139,927, published Nov. 22, 1962) or an AC current (see U.S. Pat. No. 3,933,157) is passed between the split electrodes to sense the contact resistance or impedance between the patient and the electrodes. U.S. Pat. No. 3,913,583 discloses circuitry for reducing the current passing through the patient depending upon the area of contact of the patient with a solid, patient plate, there being employed a saturable reactor in the output circuit, the impedance of which varies depending upon the sensed impedance of the contact area.
It is a primary object of this invention to provide a method and apparatus for detecting a partial loss of patient contact with a split patient electrode using capacitance measuring circuitry where the remaining area of contact is insufficient to reduce the current density at the split patient electrode to a non-dangerous level.
It is a further object of this invention to provide a power supply for contact area measurement circuitry which is isolated from ground to thereby avoid any undesirable effect on the area measurement due to grounding of the patient.
It is a further object of this invention to provide contact area measurement circuitry which operates at an amplitude which is low enough to avoid neuromuscular stimulation and interference with ECG patient monitors.
It is a further object of this invention to provide an improved method and apparatus for measuring the contact area of an active electrode with respect to a patient.
It is a further object of this invention to utilize a split capacitive electrode in conjunction with the cable connecting the split patient electrode to the generator so that the inductive impedance of the cable and the capacitive impedance of the electrode tend to cancel one another out thereby maintaining a low radio-frequency potential on the patient, the frequency of operation being such that the aforesaid effect occurs over a substantially wide range of area of patient contact with the patient electrode.
It is a further object of this invention to provide an inductance in series with the patient return cable to effect a low operating potential on the patient at predetermined frequencies of operation.
It is a further object of this invention to provide circuitry for detecting the contacting of the aforesaid insulated split electrodes with a metal surface.